Silurian Research Articles

Discussion of Searle, ‘Tectonic evolution of the Caledonian orogeny in Scotland: a review based on the timing of magmatism, metamorphism and deformation’

AbstractSearle (2022) argues, based on data from Scotland alone, that the deformation and metamorphism of the Scottish Highlands is continuously diachronous from the early Ordovician (Grampian) in the Dalradian of the Grampian Highlands to the mid-Silurian (Scandian) in the Moine of the Northwest Highlands. Necessarily, he disputes substantial offset along the Great Glen Fault (GGF). He compares the Scottish Caledonides with the Himalayas. We take the different view, which also considers evidence from along-strike in Ireland and Newfoundland, that the Grampian and Scandian orogenies were discrete events. The Grampian Orogeny involved major deformation and metamorphism across the Grampian and Northwest Highlands to the Moine Thrust Zone, whereas the Scandian Orogeny affected only the Northwest Highlands. The early Ordovician Grampian Orogeny was generated by the collision and obduction of an oceanic arc-ophiolite with and onto the Laurentian continent. The mid-Silurian Scandian Orogeny was caused by continental collision between Laurentia and Baltica. Late Silurian – early Devonian sinistral slip along the GGF emplaced the the Moine of the Northwest Highlands from a distant location to the north in the Scandian collision zone, against the Dalradian, which had not been involved in the Scandian collision. We outline several (eight) arguments that we believe contradict evidence for progressive northwestward propagation of a Himalaya-style thrust wedge from the early Ordovician to the mid-Silurian.

Recognition of sea-level fluctuations in Late Silurian to Early Permian deposits, Perlis, Malaysia

Recognition of sea-level fluctuations in Late Silurian to Early Permian deposits, Perlis, Malaysia

Petrogenesis and geodynamic implications of the Baimashan granitic complex in central Hunan, South China

The Baimashan granitic complex in central Hunan is a giant multiphase complex pluton within the South China Plate, which contains three phases (the Shuiche, Longtan‐Xiaoshajiang, and Longcangwan superunits). It is located in the Jiangnan Orogenic Belt and is of great significance for the study of the regional tectonic magmatic evolution. Herein, we utilized zircon U–Pb dating, major and trace element geochemistry, and zircon Hf isotope geochemistry for three of the Baimashan granitic complexes; data for the Longcangwan superunit were obtained from previous studies. The new zircon U–Pb ages of 418 ± 3.2 Ma and 223 ± 2.6 Ma for the Shuiche and Longtan‐Xiaoshajiang superunits, respectively, suggest the existence of Late Silurian and Late Triassic magmatism. Both superunits exhibited weak peraluminosity and high‐K calc‐alkaline and weak negative Eu anomalies, which are similar to those of I‐type granites. The Longcangwan superunit showed strong peraluminosity and high‐K calc‐alkaline and stronger negative Eu anomalies; Hf isotope and mineralogical evidence suggest that it is a fractionated I‐type granite. The zircon Hf(t) values and two‐stage depleted mantle Hf model ages revealed that the magma of the Baimashan granitic complex originates from the partial melting of metamorphic igneous rocks; the crust–mantle differentiation age in the source area was 1.5–1.7 Ga for Longtan‐Xiaoshajiang and Longcangwan and 1.7–1.9 Ga for Shuiche. The superunits formed in a post‐collisional extensional tectonic setting. In the case of the Shuiche supersuit this is related to the remote effect of the interaction of the South China Plate with the Australian‐Indian margin of Eastern Gondwana. The formation of the Longtan‐Xiaoshajiang and Longcangwan superunits is linked to the remote effect of the Indosinian and South China Plate collision and the superimposition of the subduction of the Palaeo‐Pacific Plate. The weakly fractionated I‐type granite that formed during the Caledonian in South China is closely related to scheelite mineralisation.

Two-Stage Orogenic Cycle of the Eastern Paleo-Asian Ocean from Early Palaeozoic to Early Triassic: Constraints from Magmatic Rocks of the Southeastern Central Asian Orogenic Belt

The evolution of the eastern Paleo-Asian Ocean (PAO) has controlled the formation of the southeastern Central Asian Orogenic Belt (CAOB). However, the evolution history and final closure time of the eastern PAO still remain controversial, which greatly restricts understanding of the formation process of the CAOB. To address these issues, we provide detailed zircon chronology and Hf isotope and geochemical data of Paleozoic to Triassic magmatic rocks in the southeastern CAOB. We have identified four periods of magmatism as evidenced by: Early Silurian quartz diorites (434.7 Ma), Early Devonian monzogranites (394.2 Ma), Middle Permian granites (260.2–264.5 Ma) and Late Permian-Early Triassic syenogranite (250.8–253.6 Ma). These rocks have features of low MgO and mantle-compatible elements, are enriched in Th, U, K, Pb, Sr, Zr and Hf and depleted in Nb, Ta, La, Ce, P, and Ti. The quartz diorites belong to the medium-K calc-alkaline series with εHf(t) values of −0.76 to 2.21, indicating that they may be derived from partial melting of mafic lower crust with minor contribution of mantle magma. The monzogranites and syenogranite have high Zr + Nb + Ce + Y (260–390; 261–461 ppm, respectively), total alkali contents (9.98–10.80; 8.46–9.29 wt.%, respectively), and high zircon saturation temperature (807–840; 810–885 °C). They can be classified as A-type granites. Monzogranites have εHf(t) values (between −1.20 and +3.34); hence, we believe that they were derived from the crust modified by mantle-derived fluids or melts. Syenogranite have high εHf(t) values (5.49–11.36), and we suggest that they were derived from the juvenile lower crust that originated from the depleted mantle. The granites have high Sr/Y ratios (118–257), low Y (1.42–2.82 ppm), and Yb (0.31–0.41 ppm), consistent with the features of adakite. Considering the εHf(t) isotopic values (2.99–8.50), we suggest that they originated from thickened juvenile lower crust. Combining the results from our own and previous studies, we propose a new evolution model of the eastern PAO from Paleozoic to Triassic. It can be divided into two stages: (1) Late Cambrian to Early Devonian; (2) Early Permian to Triassic. The first stage is the evolution of the Bainaimiao ocean (secondary ocean basin of the PAO), which closed in the Late Silurian and led to the Bainaimiao arc accretion to the North China Cratons. The second stage is the final closure of the eastern PAO during the Late Permian (~254 Ma).

Position of South China and Indochina along northern Gondwana margin during the Ediacaran–Silurian period

The South China Block (SCB) has often been taken as an integral member of Gondwana at its northern margin, but it remains contentious regarding when and how it joined Gondwana, its precise position in Gondwana, and what other continent(s) might have been adjacent to it at the time. This study reports new detrital zircon U-Pb and Hf isotopic data of twelve Ediacaran-Silurian siliciclastic samples from three field regions along the western margin of the SCB. They show two prevailing age populations of 810–790 Ma and 550–520 Ma, and minor ca. 2500 Ma, ca. 1850 Ma, ca. 1200 Ma and ca. 980 Ma peaks in various units. In combination with the regional lithofacies evolution, provenance analysis demonstrates that the uplifted western SCB provided a persistent supply of 810–790 Ma zircons from local Tonian granites to the Ediacaran-Silurian sediments, whereas the 550–520 Ma zircons were probably derived exotically from a magmatic arc. Provenance analysis also reveals that the SCB and Indochina have a close provenance linkage. We interpret that the exotic magmatic arc was along the northern Indian margin with the subducting lower plate carrying SCB-Indochina to eventually collide with Indian Gondwana during Cambro-Ordovician time. The uplift of the western SCB during the time likely reflects the far-field tectonic compression of the collision.

Petrogenesis and Tectonic Implications of Early Paleozoic Magmatism in Awen Gold District, South Section of the Truong Son Orogenic Belt, Laos

The Truong Son orogenic belt (TSOB) is one of the most important orogenic belts in the Indochina block. There are numerous mafic to felsic intrusions in the Early Paleozoic caused by the Tethyan orogeny. However, the tectono-magmatic evolution of the TSOB in the Early Paleozoic is still unclear. In this paper, zircon U-Pb dating, whole-rock geochemistry, and the Sr-Nd isotopic data of the Early Paleozoic magmatic rocks have been systematically investigated to explore the petrogenesis and tectonic significance of these rocks in the TSOB. Based on our new results integrated with previous geological data, four major tectono-magmatic episodes are identified. (1) The Middle Cambrian (~507 Ma) is the early stage of northward subduction of the Tamky-Phuoc Son Ocean. (2) The Early Ordovician to Middle Ordovician (483–461 Ma) is the main subduction stage of the Tamky-Phuoc Son Ocean. The intrusive rock associations imply the closure of the Tamky-Phuoc Son Ocean. (3) The Late Ordovician to Early Silurian (461–438 Ma) is the collision stage of the Kontum massif and Truong Son terrane. (4) The Early Silurian to Late Silurian (438–410 Ma) is the late stage of collision accompanied by slab roll-back.

Timing, Provenance, and Tectonic Implications of Ore-Hosting Metasedimentary Rocks in the Giant Liba Gold Deposit, West Qinling Belt, China

The closure time of the Shangdan Ocean is critical for understanding the tectonic evolution of the Proto-Tethys Ocean. However, the proposed closure time was prolonged from Ordovician to Devonian. In the present study, detrital zircon from the metasedimentary rocks of the Liba Group in the West Qinling Belt was analyzed to constrain the closure time of the Shangdan Ocean. The three youngest grains from the Liba Group yield a maximum deposition age of 418 ± 13 Ma, indicating the Middle Devonian deposition. Detrital zircon grains show two main U–Pb age peaks of 810 Ma and 440 Ma with εHf(t) values spanning from −24.3 to +8.8 and −6.3 to +4.1, respectively, suggesting that the sediments of the Liba Group were derived from both the North and South Qinling Belts. The Lower Devonian in the South Qinling Belt shows similar provenance to the Liba Group, whereas sediments from the North Qinling Belt are absent in the Silurian strata of the South Qinling Belt. From Late Silurian to Early Devonian, the tectonic setting changed from subduction to collision. This evidence consistently suggests the disappearance of the Shangdan Ocean. The noticeable decrease in magmatism from 510–420 Ma to 420–390 Ma and the shrinking of εHf(t) values from −15.5–+12.8 to −8.4–+4.2 reveal that the Shangdan Ocean, as the eastmost embranchment of the Proto-Tethys Ocean, was closed at ca. 420 Ma.

Fractures in the Niagara Escarpment in Ontario, Canada: distribution, connectivity, and geohazard implications

AbstractThe Niagara Escarpment is a geological feature located in southern Ontario, Canada, and the northeastern United States, comprising highly fractured sandstone, shale and carbonates deposited during the Ordovician and Silurian periods. Differential erosion of the strata has generated a steep cliff face which bisects the city of Hamilton, Ontario. Geological fractures are widespread in the escarpment and result in the formation of unstable blocks of rock subject to erosion through rockfall. This presents structural stability issues of concern due to the proximity of the escarpment to urban infrastructure. We quantify and analyse fracture networks in the escarpment using a combined field- and numerical-modelling-based approach. The location, orientation and aperture of fractures were documented from local outcrops using scanline and area survey methods. Clusters of poles describing the orientation of geological discontinuities were identified in spherical projections, defining three sets: (1) a sub-vertical stratabound set striking N–S, (2) a sub-vertical stratabound set striking E–W, and (3) a set parallel to horizontal sedimentary bedding planes which we infer controlled sub-vertical fracture geometry. Discrete fracture network modelling of fracture sets highlights their high degree of connectivity, and contribution to local geohazards, and quantifies their role in controlling fluid flow through escarpment strata, which is dependent on fracture aperture. Additionally, bedding planes have the potential to act as free surfaces, facilitating stress conditions in which approximately cuboid blocks are produced, and increasing the risk of rockfalls. We conclude that fractures present a first-order control on the fluid-flow properties and stability of the Niagara Escarpment.

U–Pb geochronology of Late Silurian (Wenlock to Pridoli) volcanic and sedimentary rocks, central Newfoundland Appalachians: targeting the timing of transient extension as a prelude to Devonian orogenic gold mineralization

Bimodal igneous suites and associated immature clastic sedimentary rocks are characteristic of many orogenic gold-mineralized, crustal-scale fault zones globally. In the central Newfoundland Appalachian orogen, the Rogerson Lake Conglomerate belt and Botwood basin are Late Silurian (Wenlock to Pridoli), fault-controlled sedimentary rock sequences and magmatic suites closely associated with orogenic gold mineralization; however, the spatio-temporal evolution of faulting and associated sedimentation and magmatism are not fully resolved. U–Pb zircon geochronological results were obtained by using an integrated approach employing LA-ICPMS (laser ablation-inductively coupled plasma mass spectrometry) followed by CA-ID-TIMS (chemical abrasion-isotope dilution-thermal ionization mass spectrometry) on the same detrital samples. Using this approach, a maximum depositional age for sedimentary rocks of the Rogerson Lake Conglomerate sequence is 421.9 ± 1.0 Ma (Pridoli), which confirms that they are younger than, and stratigraphically overlie, ca. 422–420 Ma igneous rocks exposed along the central Newfoundland gold belt. Towards the stratigraphic middle of the Botwood basin in north-central Newfoundland, a tuffite layer intercalated with graded siltstone produced a maximum depositional age of 427.9 ± 3.1 Ma (Wenlock; Homerian). The age of emplacement of an autobrecciated, flow-banded rhyolite dome of the Charles Lake volcanic belt along the northwestern Botwood basin is 429.3 ± 0.7 Ma (Wenlock; Homerian). The high-precision CA-ID-TIMS zircon data establish a clear link between Wenlock to Pridoli magmatism and sedimentation throughout central Newfoundland. Furthermore, these geochronological results are consistent with a structural model involving the southeastward (present-day coordinates) advancement of a transient extensional fault system across strike of the Exploits Subzone between ca. 429 and 418 Ma, with propagation along strike to the southwest (Rogerson Lake Conglomerate belt) between ca. 422 and 418 Ma. Extensional faulting may have contributed to basin formation, subsidence, and exhumation of pre-Late Silurian rocks of the Exploits Subzone. Time-transgressive, extension-related magmatism and clastic sedimentation appear to mark the transition between the Salinic and Acadian orogenic cycles along the central Newfoundland gold belt. Transient Wenlock to Pridoli lithospheric extension may have been important for increasing heat and fluid flow in the crust as a prelude to Devonian crustal thickening, fluid focussing, and orogenic gold mineralization.

ГЕОДИНАМІКА

Purpose. The objective of this study is the investigation of the Silurian-Lower Devonian (Lochkovian) carbonate-clay sedimentary complex of the southwestern slope of the East-European platform. Its formation was the result of a single cycle of geodynamic and sedimentary events in the lithological record of the southern continental margin of Baltica. Methodology. The study is based on well-logs correlation, lithological and geochemical investigations of core samples, and petrographic thin sections analysis. The obtained results were used to establish patterns of changes in the material composition of the studied strata in time and space in order to determine the basin development dynamics of the continental margin of the Eastern European platform southwestern slope in the Silurian-Early Devonian. Results. It is established that the formation of the carbonate-clay sequence represents a single sedimentary cycle and was the result of a complex of geodynamic, depositional and paleoclimatic events that took place on the Baltica southern continental margin. The Silurian period was characterized by intensive development of benthic organisms and reef structure formation in the proximal part of the basin and clay-carbonate muds enriched with dispersed organic matter in its distal part. In the Early Devonian, carbonate biogenic sedimentation continued throughout the basin. The carbonate maximum content (80–98%) proves the existence of the reef constructions in the basin sedimentary record. Lower values of carbonate content are characteristic of marls (40–55%) and biodetritic limestones (56–75%), which make up the main part of the Silurian sequence. There are no reef constructions in the section of the Lochkovian stage of Devonian, and the calcium carbonate content in the rocks varies from 45 to 83%. The content of CaCO3 in mudstones and calcareous mudstones of the Silurian varies from 5 to 15%. Originality. The applied complex of lithological, geochemical, geological-geophysical and paleogeographic investigations allowed studying the sedimentary basin dynamics in the Silurian-Early Devonian with the assessment of the possibility of participation of these strata in oil and gas hydrocarbons generation. Practical significance. The obtained results show that the carbonate-clay complex of sedimentary rocks of the Silurian and Lower Devonian (Lochkovian) of the studied region can be considered as a separate petroleum system, including source rocks, reservoir rocks and possible conventional and unconventional oil and gas accumulations.

Primitivo Hernández-Sampelayo (1880−1959): Paleozoic ironstones and fossils

The mining engineer Primitivo Hernández-Sampelayo was one of the most relevant figures in the study of the Spanish Paleozoic since Lucas Mallada’s time. After a few years working in mining companies, he entered the Geological and Mining Institute of Spain in 1914, where he worked until his retirement in 1950. The results of his studies are the large monographs on Paleozoic sedimentary ironstones (the one devoted to Galicia, 1922–1935, was published in four volumes with a total of 1,813 pp.), or the explanations to the geological map of Spain at a 1:1,000,000 scale (Cambrian System, 1934, 240 pp.; Silurian System –Ordovician included–, 1942, 848 pp.). In addition to other contributions on mineral raw materials, his papers include numerous Paleozoic regional studies, maps for the first cartographic series at a 1:50,000 scale and, above all, regional and systematic paleontological works. The latter were dedicated to Cambrian, Ordovician and Silurian trilobites, graptolites, cephalopods, archaeocyathids and ichnofossils, as well as to Carboniferous mollusks. His interest in the Paleozoic and the geology of northwestern Spain were the main subjects of his reception at the Royal Academy of Sciences of Spain (1934: Geology of Galicia) and of the inaugural address of the 1953/54 course at the same institution (Paleozoic marine life. Fossil remains). A monograph on Spanish graptolites was published in 1960 as a posthumous paper of the author. To the research work of Hernández-Sampelayo we must add his personal interest to form a museum to exhibit the mineralogical, paleontological and petrological diversity of Spain. As a result of his know-how, he established the current Geominero Museum in the great hall of the headquarters of the Geological and Mining Institute of Spain, where it was held the XIV International Geological Congress of 1926.

What to Do with Prototaxites?

Prototaxites (Nematophyta) is among the most enigmatic of terrestrial fossils. This Late Silurian–Late Devonian taxon formed multiple types of tubular filaments of varying size interwoven to produce massive treelike or columnar structures, up to more than 8 m tall and 1 m wide, making it the largest terrestrial organism of its time, towering over the surrounding vegetation. The affinities of this organism—even at the kingdom level—have been the subject of debate for over 150 years; however, the seminal work of Francis Hueber turned the tide toward a fungal interpretation that continues to remain the most widely accepted alternative. However, this characterization was made during a period in which fungal relationships and their evolutionary time lines were poorly understood. Advances in systematics and paleontology have both resulted in a substantial revision of our understanding of fungal evolutionary history. Here we reconsider Hueber’s interpretation of Prototaxites as a fungal sporocarp in the context of more recent research postdating his publication. In addition, we consider alternative suggestions by others that Prototaxites represents a lichen thallus. We argue that the weight of the evidence argues against interpretations as an agaricomycete sporocarp or ascomycete lichen association and that while the taxonomic identity remains unclear, we suggest that if Prototaxites was indeed of fungal origin, it may represent part of an extinct lineage lacking extant descendants.

Two new eurypterids (Arthropoda, Chelicerata) from the upper Silurian Yulongsi Formation of south-west China

AbstractTwo new eurypterids, a pterygotid Erettopterus qujingensis n. sp., and a slimoniid, Slimonia sp., are described from the upper Silurian (Pridolian) Yulongsi Formation of Yunnan Province, China. Erettopterus qujingensis n. sp. is characterized by several inversely curved ramus denticles and a metastoma with a deep notch in the center. The discovery not only extends the geographic extent of the genus Erettopterus and Slimonia from Euramerica to southwest China, but also gives insight into the similarity of ecosystem structures across the Silurian world.UUID: http://zoobank.org/939e82cd-58fc-415b-9209-514aecef2267

Palynology of the Freshwater East Formation (Upper Silurian, Pridoli), Pembrokeshire, South Wales, UK

Upper Silurian (Pridoli) cryptospores and trilete spores are described from the type section of the Freshwater East Formation in Pembrokeshire, South Wales. This formation is the oldest stratigraphical division of the Lower Old Red Sandstone succession south of the Ritec Fault in the Anglo-Welsh Basin. It represents an incised valley fill sequence composed predominately of red dryland alluvial sediments inter-bedded with four tidally-influenced green-grey heterothithic units. The sequence is well known for the occurrence of an early land plant Cooksonia flora. The spore assemblages are dominated by cryptospores (67%), particularly laevigate monads and dyads. Trilete spores are less abundant but are diverse in composition and provide the principal means of accurately dating the sequence. Eleven cryptospore taxa and thirty one trilete taxa are identified. Two new species Cymbohilates richardsonii and Velamisporites edwardsae are erected. The Freshwater East microflora contains a distinctive complex of Chelinospora taxa, here named the Chelinospora lavidensis assemblage. It represents a previously unrecognised assemblage within the Pridoli succession of the Anglo Welsh Basin. An accurate age for this assemblage has been determined by correlation with the Upper Silurian succession of the Cantabrian Mountains in northwest Spain, where a continuous spore succession through the Pridoli Series has independent biostratigraphical age control from chitinozoans. The Freshwater East assemblages are correlated with the upper part of the Chelinospora hemiesferica (H) Biozone, which is dated by the chitinozoans Margachitina elegans and Pseudoclathrochitina carmenchui as early mid Pridoli in age. Rare occurrences of small spiny acritarchs, sphaeromorph acritarchs and prasinophyte phycomata in two of the Freshwater East assemblages support the sedimentological data that the grey-green heterolithic beds were deposited in tidally-influenced fluvial and estuarine environments.

TRANSFORMATIVE PHASES OF SPIRITUAL PROGRESS IN ‘DASĀVATĀRA’ (TEN INCARNATIONS)

The ‘Dasāvatāra’ (Ten Incarnations) of Lord Vishnu that came to eradicate wicked forces of adharma (wickedness) and restore dharma (righteousness) shows different stages of man’s evolution from aquatic life to invertebrates, vertebrates, to modern man and finally liberation of soul all by merit. The causes and processes behind the physical evolution were not deliberated in Indic philosophy where the common ancestor is Brahma. Origin of life of all species is extra-terrestrial of lower frequencies to higher frequencies occurs automatically with each rebirth until human birth while evolution and proliferation of life of all species is only on the earth. There is no life in physical form anywhere else in the cosmos save on this planet. where life in other higher worlds exist in astral form. The ‘Dasāvatāra’ (Ten Incarnations) of Lord Vishnu reflects transformative phases of spiritual progress where an ordinary person disciplines himself to evolve as a spiritually evolved person bordering on divinity and obtain liberation or ‘moksha’. The notion of 'Dasvatra' is seen to resemble Charles Darwin's theory of evolution in several ways. The first incarnation of Vishnu as a fish resembles the Silurian Period's evolutionary origin of fish. The Geological Time Scale's history of events corresponds to the Vedic timeline of ten incarnations. The first explanation of physical evolution was given by Hinduism much before Charles Darwin did.

Fluid Inclusion Evidence for Oil Charge and Cracking in the Cambrian Longwangmiao Dolomite Reservoirs of the Central Sichuan Basin, China

The Cambrian Longwangmiao gas reservoirs in the Moxi area of the Sichuan Basin have a complex associated gas accumulation history. Based on core and thin section examination, fluid inclusion analyses and 1-D burial-thermal modelling, the diagenetic evolution and hydrocarbon accumulation processes in the Longwangmiao Formation have been reconstructed. Various diagenetic events were identified, making up a complete pore fill sequence as follows: solid bitumen/dolomite→ nonfluorescent solid bitumen → dolomite → quartz/yellow fluorescent oily bitumen → residual hole. Analyses of oil inclusions and bitumen-bearing inclusions are key to the understanding of the hydrocarbon accumulation processes. The Th values of the aqueous inclusions that are contemporaneous with hydrocarbon inclusions range from 74.3 to 214.3°C. In conjunction with burial-thermal history modelling results, the results indicate that there were two stages of oil charge and three stages of natural gas accumulation in the Longwangmiao carbonate reservoirs. The two stages of oil charge occurred in the Late Silurian and Middle Triassic, respectively. Three gas accumulation events occurred in the Middle to Late Triassic, Middle Jurassic to Early Cretaceous, and Late Cretaceous, respectively.

Layering and reservoir modeling of Silurian uppermost Tannezuft and Acacus reservoirs in the Tunisian Ghadames Basin: Inferred static model and prolific layers

Upper Silurian sediments of the uppermost Tannezuft Formation and the overlying Acacus A Unit in the Ghadames Basin of Southern Tunisia are considered as an important oil and gas target. For that, four wells, spanning these reservoirs, are evaluated herein. The study uses well logging tools via the Interactive Petrophysics (IP) software to determine hydrocarbon bearing layers. A static model is also inferred.The studied interval shows twenty-four sand bodies associated to the four petrophysical packages (T0 to T3) of the Uppermost Tannezuft and the nine petrophysical packages of the Acacus A (A1 to A9). Sand bodies show variable thicknesses (from 1.5 to 38m) and are laterally correlated following a North-West (NW) to South-East (SE) trend. They are composed of stacked thin bedded alternating sands and shales. The petrophysical packages A5 (A5-1 and A5a-2) and A9 (A9a and A9b) of the Acacus A are identified as the most interesting layers.The computed petrophysical parameters of the sand bodies reveal a clay volume ranging between 8% and 29% with an average of 22%. The effective porosity ranges from 10% to 29%. Relatively high-water saturation is depicted in most sand bodies reaching up to 63%. In addition, and due to the complex matrix composition, relatively low permeabilities characterize reservoirs zones (an average of 20mD). The integration of all data using the IP software reveals significant accumulations of gas condensates. The volumetric assessment via static modeling using the Petrel software indicates that the A9b unit is, predominantly, the most prolific reservoir.

Behavior of organic matter-hosted pores within shale gas reservoirs in response to differential tectonic deformation: Potential mechanisms and innovative conceptual models

Remarkable breakthroughs have been achieved in exploration of marine shale gas, acting as important cleaner energy resources in complex tectonic regions, encouraging us to accurately estimate different reservoir capacity of tectonically deformed gas shales. In particular, organic matter (OM) pore system, acting as the most important storage space for high-over-mature marine gas-shale reservoirs in South China has not yet been specifically targeted and comparatively studied in a regional and differential tectonic deformation regime. A set of shale drilling core samples from the Upper Silurian to Lower Ordovician Wufeng-Longmaxi Formations in the Sichuan Basin and its periphery were targeted by a multi-methodical approach utilizing organic geochemistry and mineralogical investigations, FE-SEM observation and digital image processing and extraction, and combined fluid intrusion (N2 and CO2 adsorption analysis). From undeformed shales (UDS) through slightly deformed shales (SDS) to intensely deformed shales (IDS), an obviously and progressively “Triple Jump” reduction in multi-scale pore volume and specific surface areas, and characteristic parameters for OM pores including the plane porosity (Phi) (the average dropped from 23.14% to 10.33%), the equivalent circle diameter (ECD) (the average dropped from 28.38 nm to 10.09 nm), the perimeter over area (PoA) (the average rose from 0.1126 to 0.2718), and the dominant pore diameter (DOM size) (the average dropped from 113.80 nm to 20.78 nm). The reactivation of seepage channels and intrusion of brittle minerals are proposed as two main microscopic forcing mechanisms, updated and innovative conceptual models are proposed to reconstruct an OM pore response processes under a differential tectonic deformation regime in panoramic view for Lower Paleozoic marine gas shale reservoirs of China. Thus, an evolution of spatial resolution spanning a total of three different scales respectively as structural styles, shale reservoir architectures, and microscopic petrological compositions are together portrayed.

Sedimentary and Geochemical Responses to the End Ordovician Glaciation in the Guanyinqiao Formation (Late Ordovician-Early Silurian Period) in the Sichuan Basin

Sedimentary and Geochemical Responses to the End Ordovician Glaciation in the Guanyinqiao Formation (Late Ordovician-Early Silurian Period) in the Sichuan Basin

Volcanism of the Late Silurian Eastport Formation of the Coastal Volcanic Belt, Passamaquoddy Bay, New Brunswick

This field trip is an excursion through the exquisite, nearly pristine exposures of a Silurian, felsic-dominated bimodal volcanic and sedimentary sequence exposed in the Passamaquoddy Bay area of southwestern, New Brunswick (Eastport Formation). These rocks form the northwest extension of the Coastal Volcanic Belt that extends from southwestern New Brunswick to the southern coast of Maine. The sequence is significant because it is part of a large bimodal igneous province with evidence for supervolcano-scale eruptions that began to form during the close of the Salinic Orogeny (about 424 Ma), and continued into the Acadian Orogeny (421–400 Ma). The geochemical characteristic of the rocks can be explained by extension related volcanism but the specific drivers of the extension are uncertain. The Passamaquoddy Bay sequence is 4 km thick and comprises four cycles of basaltic-rhyolitic volcanism. Basaltic volcanism typically precedes rhyolitic volcanism in Cycles 1–3. Cycle 4 represents the waning stages of volcanism and is dominated by peritidal sediments and basaltic volcanics. A spectrum of eruptive and emplacement mechanisms is represented ranging from the Hawaiian and Strombolian-type volcanism of the basaltic flows and pyroclastic scoria deposits, to highly explosive sub-Plinian to Plinian rhyolitic pyroclastic eruptions forming pyroclastic density currents (PDC) and high grade rheomorphic ignimbrites. During this field trip we will examine key exposures illustrating this spectrum of eruptive and emplacement processes, and their diagnostic characteristics, along with evidence for the interaction between mafic and felsic magmas and a variety of peperitic breccias formed as a result of emplacement of flows on wet peritidal sediments. The constraints the depositional setting and voluminous bimodal volcanism places on tectonic models will also be considered.